1. Field of the Invention
The present invention relates to a pressure sensor, and in more detail to a pressure sensor in which stress that arises other than due to the pressure to be measured is relieved and characteristic fluctuations hardly occur, a manufacturing method thereof, and an electronic component that is provided with the pressure sensor.
Priority is claimed on Japanese Patent Application No. 2008-114263, filed Apr. 24, 2008, the content of which is incorporated herein by reference.
2. Description of Related Art
A conventional semiconductor pressure sensor 110 as shown for example in FIG. 9A has gauge resistors 112 disposed in a surface 111a of a silicon substrate 111 that has a plane orientation of (100) or (110) and constitutes an electrical circuit by connecting them to a Wheatstone bridge, whereby an electrode for input/output to a pressure sensor is formed. Thereafter, using a strong alkali solution, anisotropic etching is performed from the back surface 111b of the silicon substrate 111, whereby a diaphragm 113 is formed that becomes a mechanically moving portion for detecting pressure changes. This kind of fabrication process is a method that has been generally performed in a conventional pressure sensor, but in the formation process of the diaphragm 113, due to the existence of a gradient angle on the silicon etching surface 114, there has been a limit to miniaturization of the pressure sensor. Also, although the process until the formation of the diaphragm 113 is performed in the wafer state, thereafter it is cut into small pieces as pressure sensor chips and placed in packages of resin or the like. For that reason, when mounting on a substrate like printed circuit board for placing in an electronic device, a wider mounting area has been needed.
In recent years, in order to meet the demand for miniaturization of pressure sensors, a micro pressure sensor 120 has been proposed that has a diaphragm portion 123 that forms a vacuum reference pressure chamber (cavity) 122 having a height of a few μm in a silicon substrate 121, as shown in FIG. 9B. This pressure sensor 120 is shown as a chip size package pressure sensor as disclosed for example in Patent Document “Japanese Unexamined Patent Application, First Publication No. 2007-248212”. In the structure that is disclosed in the Patent Document, members such as a ceramic package or resin package that contains the pressure sensor that is required for a conventional pressure sensor package and metal wiring that electrically connects the pressure sensor and external substrate, and leads can be completely eliminated. Also, when forming the diaphragm portion 123, it can be made small and thin because there is no need to form it in a thin manner by etching the semiconductor substrate 121 from the back side 121b of the surface 121a of the semiconductor substrate 121 on which piezoresistors 124 are arranged.
However, although the pressure sensor that has the structure illustrated in FIGS. 9A and 9B have a major merit from the viewpoint of miniaturization, a number of problems remain as shown below regarding the characteristics of the pressure sensor after mounting.
1: There is the risk of undesirable characteristic fluctuations occurring in the pressure sensor due to residual stress immediately after mounting.
2: There is the risk of undesirable characteristic fluctuations occurring in the pressure sensor under the effect of the thermal stress produced between mounting substrates by a temperature change.
3: There is the risk of undesirable characteristic fluctuations occurring in the pressure sensor owing to mechanical external factors, such as deformation, vibration, or the like of the substrate.
Regarding the abovementioned problem 1, since bonding of the mounting substrate and the joining bumps 125 is performed via a reflow process at 200 degrees C. or higher, in the process of cooling until room temperature, stress builds up due to the difference of coefficient of thermal expansion (CTE) between the mounting substrate and the pressure sensor chip. Owing to this accumulated stress, there is the risk of characteristic fluctuations occurring in the pressure sensor. Since the material that is used for the joining bumps 125 is a low melting-point material, temporal changes such as creep deformation or the like occur even in the room temperature. As a result, even if left standing at room temperature after mounting the joining bumps 125 on the substrate, the residual stress that acts on the pressure sensor changes over time. Accordingly, there is the risk of characteristic fluctuations occurring in the pressure sensor.
Regarding the abovementioned problem 2 after once cooling following mounting, due to thermal stress that is produced by changes in the surrounding temperature, there is the risk of characteristic fluctuations occurring in the pressure sensor. This is also caused by a difference of CTE between the substrate and the sensor chip.
The abovementioned problem 3 concerns characteristic fluctuations of a pressure sensor caused by stress that is applied due to mechanical factors such as substrate deformation, vibration, and dropping which a substrate in which a pressure sensor is mounted may be subjected to during use or transport.
In the case of any of problems 1 to 3, the cause of the characteristic fluctuations of the pressure sensor are due to stresses that cause pressure fluctuations other than the pressure to be measured acting on the gauge resistors on the diaphragm. Those stresses are transmitted to the pressure sensor from the mounting substrate via the bumps.
The present invention was achieved in view of the above-mentioned circumstances, and has as its object to provide a pressure sensor that can suppress stresses that cause pressure fluctuations other than the pressure to be measured and hinder output characteristic fluctuations of the pressure sensor.